byuu says:

Changelog:

  - sfc: completed BS Memory Cassette emulation (sans bugs, of course --
    testing appreciated)
  - bsnes: don't strip - on MSU1 track names in game ROM mode
    [hex_usr]

I'm going with "metadata.bml" for the flash metadata filename for the
time being, but I'll say that it's subject to change. I'll have to make
a new extension for it to be supported with bsnes.